Surface Analysis Spotlight Part 4: Concluding Perspectives on Advanced Thin Film Characterization via StrataPHI 2.0 Software

Surface Analysis Spotlight Series: Angle-Resolved X-Ray Photoelectron Spectroscopy

   by Norb Biderman

   XPS Scientist

Utilizing angle-resolved X-ray photoelectron spectroscopy (ARXPS) data, the recently released StrataPHI 2.0 software non-destructively determines the concentration versus depth profiles of the various chemical species in samples composed of thin layers, offering an alternative to the more common, destructive ion milling or “sputtering” X-ray photoelectron spectroscopy (XPS) depth profiling methods. The non-destructive approach becomes useful with 1) ultrathin films where the film thickness is thinner than the depth of damage caused by the ion beam on the order of nanometers and 2) thin films containing materials susceptible to chemical degradation due to ion-beam sputtering (e.g. the reduction of titanium dioxide to metallic titanium).

The previous Surface Analysis Spotlight Series articles dedicated to ARXPS and StrataPHI expands on their principles as well as applications:

This Surface Analysis Spotlight Series concludes with several commonly asked questions on the new StrataPHI 2.0 software:

What types of samples can be evaluated with StrataPHI 2.0?

StrataPHI is designed to analyze film stacks up to ~ 30 nm in thickness in which each thin layer in the stack is uniform in composition and thickness. Solid organic and inorganic materials of any electrical conductivity can be analyzed with StrataPHI. Film stacks such as SiO2 on a silicon substrate are found in the semiconductor and other industries where films with nanometers in thickness are made. The most appropriate samples are constructed of amorphous films, however polycrystalline films can also be analyzed. Analyzing single-crystal films may require modifications to IMFPs, material densities, and other parameters affecting the StrataPHI calculations. Given that StrataPHI currently handles discrete layers, a sample containing a layer with a non-uniform distribution of a given species with respect to depth may be modeled as several discrete layers as an approximation.

Is a prior knowledge of the sample always required to use StrataPHI 2.0?

The question of whether a measurement technique is appropriate as a “forensic” analysis method with a limited knowledge of the sample or a metrology application (e.g. in a quality-control environment) comes down to the extractable information in the data. As elaborated in the previous articles above, ARXPS and StrataPHI fall more heavily on the metrology side of this question mainly because of the requirement of inputting initial layers in the StrataPHI software prior to thickness calculation. Unknown samples consequently result in reduced confidence. However, some “forensic” information can be gleaned from the unknown sample. For example, the relative ordering of the chemical species in the depth of an unknown sample can be deduced by comparing their spectral intensities at various take-off angles. Gaining further confidence in any StrataPHI results depends upon having more knowledge of the sample history.

StrataPHI 2.0 offers the ability to combine data generated with Al Kα (XPS) and Cr Kα (HAXPES) X-rays in a single thickness calculation to analyze thick films. How can optimal photoelectron transitions be selected for acquisition?

StrataPHI includes a simulation mode where the analyst can define the composition and thickness of each layer in the model prior to starting any data acquisition. The software then calculates the predicted intensities from each layer using various Al Kα and Cr Kα transitions. Finally, the software suggests a list of the most intense photoelectron transitions based on the analyst’s inputs (Figure 9).

Ultrathin films often begin with growth of islands or may exist as islands on a substrate. Can StrataPHI still analyze such a non-continuous structure?

StrataPHI 2.0 has a new option to specify the fractional nature of a layer during the initial input in the software to simultaneously calculate the fractional coverage of the film (percent area) and its thickness (Figure 9).

Figure 9. New StrataPHI 2.0 Graphical User Interface highlighting 1) the new Simulation Mode to predict spectral intensities from a pre-defined thin-film stack structure and 2) the Fractional Coverage feature to calculate the area coverage of islands on a substrate and their thickness simultaneously.

Watch the recording of Dr. Norb Biderman’s webinar titled “StrataPHI 2.0 - Updated Software for Multi-Layered Thin-Film Structure Analysis” to learn more about non-destructive depth-profiling of multi-layered thin films using Al Kα X-rays and Cr Kα X-rays as well as fractional coverage analysis with StrataPHI 2.0.


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